In retrospect, Halo's success seems like an accidental byproduct of Microsoft's intent to deprive the Mac platform of a triple-A developer. The MacWorld Halo demo was impressive but out of all the PC developers why would Bungie be the sure thing?
the x-box was the first console with online play. x-box live debuted a year after the original console came out. for online you need a good FPS. at the time Sony and Nintendo had a lot of good devs locked up in exclusive contracts and MS needed a good game for it's new console. buying Bungie gave MS a good exclusive multi-player FPS
the x-box OS is related to Windows like iOS is related to OS X. Bungie was already making Windows games so the costs to port to x-box would be minimal
and profit. one reason why nintendo has been ridiculously profitable until recently was that it produced most of the big hits for it's own console